Method and apparatus for atrial arrhythmia episode detection

ABSTRACT

A method and implantable medical device for determining noise in response to a cardiac signal that includes sensing the cardiac signal, determining a sensing window in response to the sensed cardiac signal, the sensing window comprising a first portion and a second portion, determining a first derivative signal in response to the sensed cardiac signal within only one of the first portion and the second portion of the sensing window, determining a second derivative signal in response to the sensed cardiac signal within the one of the first portion and the second portion of the sensing window, determining whether an amplitude of the second derivative signal satisfies an amplitude threshold, and determining noise in response to the amplitude of the second derivative signal satisfying the amplitude threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/117,785, filed on Feb. 18, 2015, incorporated herein by reference inits entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable cardiac medical devicesand, in particular, to a method for and apparatus for detecting atrialtachyarrhythmia episodes in an implantable cardiac medical device.

BACKGROUND

During normal sinus rhythm (NSR), the heart beat is regulated byelectrical signals produced by the sino-atrial (SA) node located in theright atrial wall. Each atrial depolarization signal produced by the SAnode spreads across the atria, causing the depolarization andcontraction of the atria, and arrives at the atrioventricular (A-V)node. The A-V node responds by propagating a ventricular depolarizationsignal through the bundle of His of the ventricular septum andthereafter to the bundle branches and the Purkinje muscle fibers of theright and left ventricles.

Atrial tachyarrhythmia includes the disorganized form of atrialfibrillation and varying degrees of organized atrial tachycardia,including atrial flutter. Atrial fibrillation (AF) occurs because ofmultiple focal triggers in the atrium or because of changes in thesubstrate of the atrium causing heterogeneities in conduction throughdifferent regions of the atria. The ectopic triggers can originateanywhere in the left or right atrium or pulmonary veins. The AV nodewill be bombarded by frequent and irregular atrial activations but willonly conduct a depolarization signal when the AV node is not refractory.The ventricular cycle lengths will be irregular and will depend on thedifferent states of refractoriness of the AV-node.

In the past, atrial arrhythmias have been largely undertreated due tothe perception that these arrhythmias are relatively benign. As moreserious consequences of persistent atrial arrhythmias have come to beunderstood, such as an associated risk of relatively more seriousventricular arrhythmias and stroke, there is a growing interest inmonitoring and treating atrial arrhythmias.

Methods for discriminating arrhythmias that are atrial in origin fromarrhythmias originating in the ventricles have been developed for use indual chamber implantable devices wherein both an atrial EGM signal and aventricular EGM signal are available. Discrimination of arrhythmias canrely on event intervals (PP intervals and RR intervals), event patterns,and EGM morphology. Such methods have been shown to reliablydiscriminate ventricular arrhythmias from supra-ventricular arrhythmias.In addition, such methods have been developed for use in single chamberimplantable devices, subcutaneous implantable devices, and externalmonitoring devices, where an adequate atrial EGM signal havingacceptable signal-to-noise ratio is not always available for use indetecting and discriminating atrial arrhythmias.

Occasionally, false detection of atrial fibrillation may occur in asubcutaneous device during runs of ectopic rhythm with irregularcoupling intervals or underlying sinus variability/sick sinus. Inaddition, false detection of atrial tachycardia may occur in asubcutaneous device during ectopy and regular normal sinus rhythm.Therefore, what is needed is a method for improving detection of atrialtachyarrhythmia to reduce false detection in a medical device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary medical device fordetecting an arrhythmia according to an embodiment of the presentdisclosure.

FIG. 2 is a functional schematic diagram of the medical device of FIG.1.

FIG. 3 is a flowchart of a method for detecting an atrial arrhythmiaaccording to an embodiment of the disclosure.

FIG. 4 is a schematic diagram of detecting an atrial arrhythmiaaccording to an embodiment of the disclosure.

FIG. 5 is a flowchart of a method of detecting an atrial arrhythmia in amedical device according to an embodiment of the disclosure.

FIG. 6 is a schematic diagram of detecting an atrial arrhythmia in amedical device, according to an embodiment of the disclosure.

FIG. 7 is a flowchart of a method of determining an atrial arrhythmiaaccording to an embodiment of the disclosure.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments for carrying out the methods described herein. It isunderstood that other embodiments may be utilized without departing fromthe scope of the disclosure.

In various embodiments, ventricular signals are used for determiningsuccessive ventricular cycle lengths for use in detecting atrialarrhythmias. The atrial arrhythmia detection methods do not require anelectrode positioned within the atrium as an atrial signal source todirectly sense the atrial signal within the heart; i.e., the device maybe a single chamber device having an electrode positioned only withinthe ventricle, or a subcutaneous device having no electrode positionedwithin the heart. The methods presented herein may be embodied insoftware, hardware or firmware in implantable or external medicaldevices. Such devices include implantable monitoring devices havingcardiac EGM/ECG monitoring capabilities and associated EGM/ECG senseelectrodes, which may be intracardiac, epicardial, or subcutaneouselectrodes.

The methods described herein can also be incorporated in implantablemedical devices having therapy delivery capabilities, such as singlechamber or bi-ventricular pacing systems or ICDs that sense the R-wavesin the ventricles and deliver an electrical stimulation therapy to theventricles. The atrial arrhythmia detection methods presently disclosedmay also be incorporated in external monitors having ECG electrodescoupled to the patient's skin to detect R-waves, e.g. Holter monitors,or within computerized systems that analyze pre-recorded ECG or EGMdata. Embodiments may further be implemented in a patient monitoringsystem, such as a centralized computer system which processes data sentto it by implantable or wearable monitoring devices, includingsubcutaneous devices having loop recorders.

FIG. 1 is a schematic diagram of an exemplary medical device fordetecting an arrhythmia according to an embodiment of the presentdisclosure. As illustrated in FIG. 1, a medical device according to anembodiment of the present disclosure may be in the form of animplantable cardioverter defibrillator (ICD) 10 a connector block 12that receives the proximal ends of a right ventricular lead 16, a rightatrial lead 15 and a coronary sinus lead 6, used for positioningelectrodes for sensing and stimulation in three or four heart chambers.Right ventricular lead 16 is positioned such that its distal end is inthe right ventricle for sensing right ventricular cardiac signals anddelivering pacing or shocking pulses in the right ventricle. For thesepurposes, right ventricular lead 16 is equipped with a ring electrode24, an extendable helix electrode 26 mounted retractably within anelectrode head 28, and a coil electrode 20, each of which are connectedto an insulated conductor within the body of lead 16. The proximal endof the insulated conductors are coupled to corresponding connectorscarried by bifurcated connector 14 at the proximal end of lead 16 forproviding electrical connection to the ICD 10. It is understood thatalthough the device illustrated in FIG. 1 is a dual chamber device,other devices such as single chamber devices may be utilized to performthe technique of the present disclosure described herein.

The right atrial lead 15 is positioned such that its distal end is inthe vicinity of the right atrium and the superior vena cava. Lead 15 isequipped with a ring electrode 21 and an extendable helix electrode 17,mounted retractably within electrode head 19, for sensing and pacing inthe right atrium. Lead 15 is further equipped with a coil electrode 23for delivering high-energy shock therapy. The ring electrode 21, thehelix electrode 17 and the coil electrode 23 are each connected to aninsulated conductor with the body of the right atrial lead 15. Eachinsulated conductor is coupled at its proximal end to a connectorcarried by bifurcated connector 13.

The coronary sinus lead 6 is advanced within the vasculature of the leftside of the heart via the coronary sinus and great cardiac vein. Thecoronary sinus lead 6 is shown in the embodiment of FIG. 1 as having adefibrillation coil electrode 8 that may be used in combination witheither the coil electrode 20 or the coil electrode 23 for deliveringelectrical shocks for cardioversion and defibrillation therapies. Inother embodiments, coronary sinus lead 6 may also be equipped with adistal tip electrode and ring electrode for pacing and sensing functionsin the left chambers of the heart. The coil electrode 8 is coupled to aninsulated conductor within the body of lead 6, which provides connectionto the proximal connector 4.

The electrodes 17 and 21 or 24 and 26 may be used as true bipolar pairs,commonly referred to as a “tip-to-ring” configuration. Further,electrode 17 and coil electrode 20 or electrode 24 and coil electrode 23may be used as integrated bipolar pairs, commonly referred to as a“tip-to-coil” configuration. In accordance with the invention, ICD 10may, for example, adjust the electrode configuration from a tip-to-ringconfiguration, e.g., true bipolar sensing, to a tip-to-coilconfiguration, e.g., integrated bipolar sensing, upon detection ofoversensing in order to reduce the likelihood of future oversensing. Inother words, the electrode polarities can be reselected in response todetection of oversensing in an effort to reduce susceptibility ofoversensing. In some cases, electrodes 17, 21, 24, and 26 may be usedindividually in a unipolar configuration with the device housing 11serving as the indifferent electrode, commonly referred to as the “can”or “case” electrode.

The device housing 11 may also serve as a subcutaneous defibrillationelectrode in combination with one or more of the defibrillation coilelectrodes 8, 20 or 23 for defibrillation of the atria or ventricles. Itis recognized that alternate lead systems may be substituted for thethree lead system illustrated in FIG. 1. While a particularmulti-chamber ICD and lead system is illustrated in FIG. 1,methodologies included in the present invention may adapted for use withany single chamber, dual chamber, or multi-chamber ICD or pacemakersystem, subcutaneous implantable device, or other internal or externalcardiac monitoring device.

ICD 10 may alternatively be configured as a subcutaneous device havingsensing or pacing electrodes incorporated on the housing 11 of thedevice in which case transvenous leads are not required. A subcutaneousdevice may be coupled to a lead tunneled subcutaneously or submuscularlyfor delivering transthoracic pacing pulses and/or sensing ECG signals.An exemplary subcutaneous device is described in commonly assigned U.S.patent application Ser. Nos. 14/604,111 and 14/604,260, bothincorporated herein by reference in their entireties. The techniquesdescribed herein can also be implemented in an external device, e.g.including patch electrodes and optionally another physiological sensorif desired, that can sense variable parameters as described herein.

FIG. 2 is a functional schematic diagram of the medical device ofFIG. 1. This diagram should be taken as exemplary of the type of devicewith which the invention may be embodied and not as limiting. Thedisclosed embodiment shown in FIG. 2 is a microprocessor-controlleddevice, but the methods of the present invention may also be practicedwith other types of devices such as those employing dedicated digitalcircuitry.

With regard to the electrode system illustrated in FIG. 1, ICD 10 isprovided with a number of connection terminals for achieving electricalconnection to the leads 6, 15, and 16 and their respective electrodes. Aconnection terminal 311 provides electrical connection to the housing 11for use as the indifferent electrode during unipolar stimulation orsensing. The connection terminals 320, 313, and 318 provide electricalconnection to coil electrodes 20, 8 and 23 respectively. Each of theseconnection terminals 311, 320, 313, and 318 are coupled to the highvoltage output circuit 234 to facilitate the delivery of high energyshocking pulses to the heart using one or more of the coil electrodes 8,20, and 23 and optionally the housing 11.

The connection terminals 317 and 321 provide electrical connection tothe helix electrode 17 and the ring electrode 21 positioned in the rightatrium. The connection terminals 317 and 321 are further coupled to anatrial sense amplifier 204 for sensing atrial signals such as P-waves.The connection terminals 326 and 324 provide electrical connection tothe helix electrode 26 and the ring electrode 24 positioned in the rightventricle. The connection terminals 326 and 324 are further coupled to aventricular sense amplifier 200 for sensing ventricular signals. Theatrial sense amplifier 204 and the ventricular sense amplifier 200preferably take the form of automatic gain controlled amplifiers withadjustable sensitivity. In accordance with the invention, ICD 10 and,more specifically, microprocessor 224 automatically adjusts thesensitivity of atrial sense amplifier 204, ventricular sense amplifier200 or both in response to detection of oversensing in order to reducethe likelihood of oversensing. Ventricular sense amplifier 200 andatrial sense amplifier 204 operate in accordance with originallyprogrammed sensing parameters for a plurality of cardiac cycles, andupon detecting oversensing, automatically provides the corrective actionto avoid future oversensing. In this manner, the adjustments provided byICD 10 to amplifiers 200 and 204 to avoid future oversensing are dynamicin nature. Particularly, microprocessor 224 increases a sensitivityvalue of the amplifiers, thus reducing the sensitivity, when oversensingis detected. Atrial sense amplifier 204 and ventricular sense amplifier200 receive timing information from pacer timing and control circuitry212.

Specifically, atrial sense amplifier 204 and ventricular sense amplifier200 receive blanking period input, e.g., ABLANK and VBLANK,respectively, which indicates the amount of time the electrodes are“turned off” in order to prevent saturation due to an applied pacingpulse or defibrillation shock. As will be described, the blankingperiods of atrial sense amplifier 204 and ventricular sense amplifier200 and, in turn, the blanking periods of sensing electrodes associatedwith the respective amplifiers may be automatically adjusted by ICD 10to reduce the likelihood of oversensing. The general operation of theventricular sense amplifier 200 and the atrial sense amplifier 204 maycorrespond to that disclosed in U.S. Pat. No. 5,117,824, by Keimel, etal., incorporated herein by reference in its entirety. Whenever a signalreceived by atrial sense amplifier 204 exceeds an atrial sensitivity, asignal is generated on the P-out signal line 206. Whenever a signalreceived by the ventricular sense amplifier 200 exceeds a ventricularsensitivity, a signal is generated on the R-out signal line 202.

Switch matrix 208 is used to select which of the available electrodesare coupled to a wide band amplifier 210 for use in digital signalanalysis. Selection of the electrodes is controlled by themicroprocessor 224 via data/address bus 218. The selected electrodeconfiguration may be varied as desired for the various sensing, pacing,cardioversion and defibrillation functions of the ICD 10. Specifically,microprocessor 224 may modify the electrode configurations based ondetection of oversensing due to cardiac or non-cardiac origins. Upondetection of R-wave oversensing, for example, microprocessor 224 maymodify the electrode configuration of the right ventricle from truebipolar sensing, e.g., tip-to-ring, to integrated bipolar sensing, e.g.,tip-to-coil.

Signals from the electrodes selected for coupling to bandpass amplifier210 are provided to multiplexer 220, and thereafter converted tomulti-bit digital signals by A/D converter 222, for storage in randomaccess memory 226 under control of direct memory access circuit 228 viadata/address bus 218. Microprocessor 224 may employ digital signalanalysis techniques to characterize the digitized signals stored inrandom access memory 226 to recognize and classify the patient's heartrhythm employing any of the numerous signal processing methodologiesknown in the art. An exemplary tachyarrhythmia recognition system isdescribed in U.S. Pat. No. 5,545,186 issued to Olson et al, incorporatedherein by reference in its entirety.

Upon detection of an arrhythmia, an episode of EGM data, along withsensed intervals and corresponding annotations of sensed events, arepreferably stored in random access memory 226. The EGM signals storedmay be sensed from programmed near-field and/or far-field sensingelectrode pairs. Typically, a near-field sensing electrode pair includesa tip electrode and a ring electrode located in the atrium or theventricle, such as electrodes 17 and 21 or electrodes 26 and 24. Afar-field sensing electrode pair includes electrodes spaced furtherapart such as any of: the defibrillation coil electrodes 8, 20 or 23with housing 11; a tip electrode 17 or 26 with housing 11; a tipelectrode 17 or 26 with a defibrillation coil electrode 20 or 23; oratrial tip electrode 17 with ventricular ring electrode 24. The use ofnear-field and far-field EGM sensing of arrhythmia episodes is describedin U.S. Pat. No. 5,193,535, issued to Bardy, incorporated herein byreference in its entirety. Annotation of sensed events, which may bedisplayed and stored with EGM data, is described in U.S. Pat. No.4,374,382 issued to Markowitz, incorporated herein by reference in itsentirety.

The telemetry circuit 330 receives downlink telemetry from and sendsuplink telemetry to an external programmer, as is conventional inimplantable anti-arrhythmia devices, by means of an antenna 332. Data tobe uplinked to the programmer and control signals for the telemetrycircuit are provided by microprocessor 224 via address/data bus 218. EGMdata that has been stored upon arrhythmia detection or as triggered byother monitoring algorithms may be uplinked to an external programmerusing telemetry circuit 330. Received telemetry is provided tomicroprocessor 224 via multiplexer 220. Numerous types of telemetrysystems known in the art for use in implantable devices may be used.

The remainder of the circuitry illustrated in FIG. 2 is an exemplaryembodiment of circuitry dedicated to providing cardiac pacing,cardioversion and defibrillation therapies. The pacer timing and controlcircuitry 212 includes programmable digital counters which control thebasic time intervals associated with various single, dual ormulti-chamber pacing modes or anti-tachycardia pacing therapiesdelivered in the atria or ventricles. Pacer circuitry 212 alsodetermines the amplitude of the cardiac pacing pulses under the controlof microprocessor 224.

During pacing, escape interval counters within pacer timing and controlcircuitry 212 are reset upon sensing of R-waves or P-waves as indicatedby signals on lines 202 and 206, respectively. In accordance with theselected mode of pacing, pacing pulses are generated by atrial paceroutput circuit 214 and ventricular pacer output circuit 216. The paceroutput circuits 214 and 216 are coupled to the desired electrodes forpacing via switch matrix 208. The escape interval counters are resetupon generation of pacing pulses, and thereby control the basic timingof cardiac pacing functions, including anti-tachycardia pacing.

The durations of the escape intervals are determined by microprocessor224 via data/address bus 218. The value of the count present in theescape interval counters when reset by sensed R-waves or P-waves can beused to measure R-R intervals and P-P intervals for detecting theoccurrence of a variety of arrhythmias.

The microprocessor 224 includes associated read-only memory (ROM) inwhich stored programs controlling the operation of the microprocessor224 reside. A portion of the random access memory (RAM) 226 may beconfigured as a number of recirculating buffers capable of holding aseries of measured intervals for analysis by the microprocessor 224 forpredicting or diagnosing an arrhythmia. In response to the detection oftachycardia, anti-tachycardia pacing therapy can be delivered by loadinga regimen from microprocessor 224 into the pacer timing and controlcircuitry 212 according to the type of tachycardia detected. In theevent that higher voltage cardioversion or defibrillation pulses arerequired, microprocessor 224 activates the cardioversion anddefibrillation control circuitry 230 to initiate charging of the highvoltage capacitors 246 and 248 via charging circuit 236 under thecontrol of high voltage charging control line 240. The voltage on thehigh voltage capacitors is monitored via a voltage capacitor (VCAP) line244, which is passed through the multiplexer 220. When the voltagereaches a predetermined value set by microprocessor 224, a logic signalis generated on the capacitor full (CF) line 254, terminating charging.The defibrillation or cardioversion pulse is delivered to the heartunder the control of the pacer timing and control circuitry 212 by anoutput circuit 234 via a control bus 238. The output circuit 234determines the electrodes used for delivering the cardioversion ordefibrillation pulse and the pulse wave shape.

In one embodiment, the ICD 10 may be equipped with a patientnotification system 150. Any patient notification method known in theart may be used such as generating perceivable twitch stimulation or anaudible sound. A patient notification system may include an audiotransducer that emits audible sounds including voiced statements ormusical tones stored in analog memory and correlated to a programming orinterrogation operating algorithm or to a warning trigger event asgenerally described in U.S. Pat. No. 6,067,473 issued to Greeninger etal., incorporated herein by reference in its entirety.

FIG. 3 is a flowchart of a method for detecting an atrial arrhythmiaaccording to an embodiment of the disclosure. As illustrated in FIG. 3,in order to determine whether a sensed cardiac signal is an atrialtachycardia event, the device determines whether the cardiac signalcontains a P-wave portion, the results of which are utilized to augmentan atrial tachycardia determination process. For example, thedetermination as to whether a P-wave is detected may be utilized toaugment detection of atrial arrhythmias based on the irregularity ofventricular cycles having RR intervals that exhibit discriminatorysignatures when plotted in a Lorenz scatter plot, such as is generallydisclosed by Ritscher et al. in U.S. Pat. No. 7,031,765, or in U.S. Pat.No. 8,639,316 to Sarkar, both incorporated herein by reference in theirentireties. Other atrial arrhythmia determination methods are generallydisclosed by Sarkar, et al. in U.S. Pat. No. 7,623,911 and in U.S. Pat.No. 7,537,569, and by Houben in U.S. Pat. No. 7,627,368, all of whichpatents are also incorporated herein by reference in their entireties.

According to one embodiment, for example, during determination of signalcharacteristics for augmenting atrial tachycardia detection, the devicesenses the cardiac signal and identifies R-waves in response to thesensed cardiac signal using any known cardiac signal sensing anddetection scheme, such as that disclosed in U.S. Pat. No. 5,117,824, byKeimel, et al., for example, described above and incorporated herein byreference in its entirety. Upon detection of an R-wave associated withthe sensed cardiac signal, Block 300, the device determines whether theR-wave satisfies one or more RR-interval parameters, Block 302,described below. If the RR-interval parameter or parameters are notsatisfied, No in Block 302, the device waits for the next sensed R-wave,Block 300 and the process Block 300-302 is repeated using the nextR-wave. If the RR-interval parameter or parameters are satisfied, Yes inBlock 302, the device determines a P-wave window associated with theR-wave, Block 304, as described below.

Upon determination of the P-wave window, the device determines whether apredetermined number of R-waves have been identified, Block 306. Thepredetermined number of R-waves required to satisfy the determination inBlock 306 may be set as one or more R-waves, and according to oneembodiment is set as four R-waves for example. If the predeterminednumber of R-waves have not been identified and therefore a next R-waveis needed, Yes in Block 306, the device waits for the next sensedR-wave, Block 300 and the process Block 300-306 is repeated using thenext R-wave. If the predetermined number of R-waves have been identifiedand therefore a next R-wave is not needed, No in Block 306, the devicedetermines P-wave evidence, Block 308, described below, and utilizes thedetermined P-wave evidence to augment atrial arrhythmia detection, Block310, as described, for example, in commonly assigned U.S. patentapplication Ser. No. ______ (Attorney Docket No. C00002902.USU5),incorporated herein by reference in it's entirety.

FIG. 4 is a schematic diagram of detecting an atrial arrhythmiaaccording to an embodiment of the disclosure. As illustrated in FIGS. 3and 4, in order to determine whether a sensed R-wave 320 satisfies theRR-interval parameters in Block 302, the device determines whether an RRinterval 322 extending between the current R-wave 320 and a previoussensed R-wave 324 is greater than an interval threshold, such as 780 msfor example. If the RR interval 322 is not greater than the intervalthreshold, the RR-interval parameter is not satisfied, No in Block 302,and the process is repeated with the next RR interval 326. If the RRinterval 322 is greater than the interval threshold, the RR intervalparameter is satisfied, Yes in Block 302.

According to another embodiment, additional RR interval parameters mayalso be included in the determination as to whether the RR intervalparameters have been satisfied in Block 302. For example, using R wave326 as an example, in addition to the determination of whether theassociated RR interval 340 satisfies the RR interval threshold, thedevice may also compare the RR interval 340 associated with the currentR wave 326 with one or more previously determined RR intervals, such asinterval 322 for example, and determine whether a relative changeassociated with the current RR-interval 340 is greater than a changethreshold, such as 100 ms, for example. If the relative changeassociated with the current RR-interval is not greater than the changethreshold, the RR interval parameter is not satisfied in Block 302. Ifthe relative change associated with the current RR interval is greaterthan the change threshold, the RR-interval parameter is satisfied inBlock 302.

In this way, if one of the RR intervals parameters are not satisfied, noP-wave window determination is made, and the process is repeated withthe next R wave. If the RR interval parameter or one of the RR intervalparameters are satisfied, the RR interval parameter is satisfied inBlock 302, and the device determines a P wave window 328 associated withthe R-wave 320 for determining whether the R wave 320 includes anassociated P-wave. For example, in order to determine the P wave window328, the device determines a P-wave window start point 330 located apredetermined distance 332 prior to the R-wave, such as 620 ms forexample, and a P wave window endpoint 334 is located at a predetermineddistance 336 subsequent to the P wave start point 330, such as 600 ms,for example, so that the P wave window 328 extends 600 ms between the Pwave start point 330 and the P wave endpoint 334. Each time a P wavewindow 328 is determined, a P wave counter is updated by one, until thepredetermined number of P wave windows are identified, such as four Pwave windows, for example.

FIG. 5 is a flowchart of a method of detecting an atrial arrhythmia in amedical device according to an embodiment of the disclosure. In responseto the predetermined number of P-waves being identified, No in Block 306of FIG. 3, the device determines P-wave evidence for determining whethera P-wave is likely detected, Block 308, and utilizes the determinedP-wave evidence to augment atrial arrhythmia detection, Block 310,described, for example, in commonly assigned U.S. patent applicationSer. No. ______ (Attorney Docket No. C00002902.USU5), incorporatedherein by reference in it's entirety. As illustrated in FIG. 5, duringthe determination of P-wave evidence, the device determines acharacteristic P-wave in response to the current determined P-waves,Block 360. For example, according to one embodiment, the devicedetermines an average P-wave from the four determined P-waves that isidentified as the characteristic P-wave. The associated P-wave window isthen divided into a baseline portion, Block 362, and a P-wave portion,Block 364, and determines signal characteristics, Block 366, for one orboth of the baseline window and the P-wave window. A determination isthen made, based on the determined signal characteristics, whether thecharacteristic P-wave is confirmed as being a P-wave, Block 368.

If the characteristic P-wave is not confirmed as being a P-wave, No inBlock 368, the device waits for the next predetermined number of P-wavesto be identified, Yes in Block 306 of FIG. 3, and the process, Blocks360-368, is repeated using the next identified P-waves. If thecharacteristic P-wave is confirmed as being a P-wave, Yes in Block 368,the device utilizes the determination of a P-wave being present toaugment atrial arrhythmia detection, Block 370, as described forexample, in commonly assigned U.S. patent application Ser. No. ______(Attorney Docket No. C00002902.USU5), incorporated herein by referencein it's entirety.

FIG. 6 is a schematic diagram of detecting an atrial arrhythmia in amedical device, according to an embodiment of the disclosure. Asillustrated in FIGS. 5 and 6, in order to determine P-wave evidence(Block 308 of FIG. 3), the device determines a characteristic P-wave 400having a characteristic P wave window 402 determined by averaging thedetermined four P-wave windows, as described above. The device dividesthe P-wave window 402 into a baseline portion 404, extending from theP-wave window start point 406 to a midpoint of the window 408, and aP-wave portion 410, extending from the midpoint of the window 408 to aP-wave window endpoint 412. The device determines a first derivative ofthe P-wave signal 414 and a second derivative of the p-wave signal 416,and determines corresponding second derivative values 420 associatedwith positive going zero crossings 418 of the first derivative signal414 within the baseline portion 404 of the first derivative signalwindow 402. In one embodiment, the first derivative of the P wave signalcan be computed as the difference between points separated by eightsamples, and the second derivative can be computed as the differencebetween points separated by four sample in the first derivative.

The device determines the maximum amplitude of the second derivativevalues 420 associated with the positive going zero crossings 418, andthe determined maximum amplitude value is then used to generate a firstthreshold 422 for evaluating the second derivative P-wave signal 416within the P-wave portion 410 of the second derivative window 402.According to one embodiment, the threshold 422 is set as a multiple ofthe maximum of the second derivative values 420, such as twice themaximum of the second derivative values 420, for example.

In the same way, the device determines a corresponding second derivativevalue 426 for each negative going zero crossing 424 of the derivativesignal 414 within the baseline portion 404 of the window 402. A minimumamplitude of the second derivative values 426 associated with thenegative going first derivative zero crossings 424 is determined, andthe determined minimum amplitude value is used to generate a secondthreshold 428 for evaluating the second derivative P-wave signal 416within the P-wave portion 410 of the window 402. According to oneembodiment, the threshold 428 is set as a multiple of the minimum of thesecond derivative values 426, such as twice the minimum of the secondderivative values 426, for example.

Using the first threshold 422 determined in response to the determinedmaximum of the second derivative values 420, the device determines, foreach positive going zero crossing 430 of the first derivative signalwithin the P-wave portion 410 of the first derivative window, acorresponding amplitude 432 of the second derivative signal within theP-wave portion 410 of the corresponding second derivative signal 416.The device compares the resulting maximum amplitudes 432 of the secondderivative signal 416 signal within the P-wave portion 410 of the window402 to the first threshold 422. Similarly, using the second threshold422 determined in response to the determined minimum of the secondderivative values 420, the device compares, for one or more negativegoing zero crossing 434 of the first derivative signal 414, thecorresponding minimum amplitude 436 of the second derivative signal 416signal within the P-wave portion 410 of the window 402 to the secondthreshold 428.

A P-wave is determined to have occurred, Yes in Block 368 of FIG. 5, ifeither the number of maximum amplitudes 432 determined to be greaterthan or equal to the first threshold 422 is equal to one, or the numberof minimum amplitudes 432 determined to be less than or equal to thesecond threshold 428 is equal to one. If both the number of maximumamplitudes 432 determined to be greater than or equal to the firstthreshold 422 and the number of minimum amplitudes 432 determined to beless than or equal to the second threshold 428 is not equal to one, aP-wave is not determined to have occurred, No in Block 368 of FIG. 5.The result of the determination of whether a P-wave is identified isthen used during the determination of an atrial arrhythmia event, asdescribed for example, in commonly assigned U.S. patent application Ser.No. ______ (Attorney Docket No. C00002902.USU5), incorporated herein byreference in it's entirety.

FIG. 7 is a flowchart of a method of determining an atrial arrhythmiaaccording to an embodiment of the disclosure. As illustrated in FIGS. 6and 7, during detection of P-wave evidence, the device may alsodetermine that the event is associated with other events, such as noise,for example. During determination of signal characteristics (Block 366of FIG. 5), the device may also determine a noise event is occurring inresponse to any one of a predetermined conditions being met. Forexample, in order to determine whether a noise event is occurring, thedevice may determine the amplitudes of the second derivative signallocated at both the positive going zero crossing and the negative goingzero crossings of the first derivative signal 414 within the baselineportion 404 of the window 402, Block 600, and determine whether both amaximum amplitude 460 of the second derivative signal 416 at a positivezero crossing of the first derivative signal 414 within the baselineportion 404 of the window 402 and a minimum amplitude 462 of the secondderivative signal 416 at a negative zero crossing of the firstderivative signal 414 within the baseline portion 404 of the window 402satisfy a first amplitude threshold, Block 602, such as the maximumamplitude being greater than 16 microvolts and the minimum amplitudebeing less than −16 microvolts, for example.

If the first amplitude threshold is satisfied, Yes in Block 602, noiseis identified for the characteristic P-wave 400, Block 604. If the firstamplitude threshold is not satisfied, No in Block 602, the device maydetermine other conditions for indicating noise, such as determiningwhether either a maximum amplitude 460 of the second derivative signal416 at a positive zero crossing of the first derivative signal 414within the baseline portion 404 of the window 402 or a minimum amplitude462 of the second derivative signal 416 at a negative zero crossing ofthe first derivative signal 414 within the baseline portion 404 of thewindow 402 satisfy a second amplitude threshold, Block 606, such as themaximum amplitude being greater than 49 microvolts or the minimumamplitude being less than −49 microvolts, for example.

If the second amplitude threshold is satisfied, Yes in Block 606, noiseis identified for the characteristic P-wave 400, Block 604. If thesecond amplitude threshold is not satisfied, No in Block 606, the devicemay determine the number of positive going zero crossings of the firstderivative signal 414 within the baseline portion 404 of the window 402whose corresponding amplitude 460 of the second derivative signal 416 isgreater than a maximum amplitude threshold 464, such as 16 microvolts,for example, and the number of negative going zero crossings of thefirst derivative signal 414 within the baseline portion 404 of thewindow 402 whose corresponding minimum amplitude 462 of the secondderivative signal 416 is less than a minimum amplitude threshold 466,such as −16 microvolts for example, Block 608. A determination is thenmade as to whether an amplitude threshold is satisfied, Block 610, andif the amplitude threshold is satisfied, Yes in Block 610, noise isidentified, Block 604. For example, according to one embodiment, thedevice determines whether the amplitude threshold is satisfied in Block610 by determining whether a sum of both the number of positive goingzero crossings of the first derivative signal 414 within the baselineportion 404 of the window 402 whose corresponding amplitude 460 of thesecond derivative signal 416 is greater than the maximum amplitudethreshold 464 and the number of negative going zero crossings of thefirst derivative signal 414 within the baseline portion 404 of thewindow 402 whose corresponding minimum amplitude 462 of the secondderivative signal 416 is less than the minimum amplitude threshold 466being equal to a predetermined number, such as 3 for example.

If the amplitude threshold is not satisfied, No in Block 610, the devicemay determine the number of positive zero crossings within the baselineportion 404 of the window 402 and the number of negative zero crossingswithin the baseline portion 404 of the window 402, Block 612. Adetermination is made as to whether a combined amplitude threshold and abaseline crossing threshold is satisfied, Block 614, by determining, forexample, whether both the sum of the number of positive going zerocrossings of the first derivative signal 414 within the baseline portion404 of the window 402 whose corresponding amplitude 460 of the secondderivative signal 416 is greater than the maximum amplitude threshold464 and the number of negative going zero crossings of the firstderivative signal 414 within the baseline portion 404 of the window 402whose corresponding minimum amplitude 462 of the second derivativesignal 416 is less than the minimum amplitude threshold 464 is equal toa predetermined number, such as three for example, and the sum of thenumber of positive zero crossings within the baseline portion 404 of thewindow 402 and the number of negative zero crossings within the baselineportion 404 of the window 402 is within a predetermined range, such asgreater than four and less than ten, for example.

If the combined amplitude threshold and baseline crossing threshold issatisfied, Yes in Block 614, a noise event is identified, Block 604. Ifthe combined amplitude threshold and a baseline crossing threshold issatisfied, No in Block 614, the device may determine the number ofpositive going zero crossings and the number of negative going zerocrossings of the first derivative signal 414 within the P-wave portion410 of the window 302, Block 616, and determine whether a zero crossingsthreshold has been satisfied, Block 618, by determining whether a sum ofthe determined number of positive going zero crossings and the number ofnegative going zero crossings of the first derivative signal 414 isgreater than four, for example.

If the zero crossings threshold has been satisfied, Yes in Block 618, anoise event is determined, Block 604. If the zero crossings thresholdhas not been satisfied, No in Block 618, the device may determine thenumber of amplitudes 460 of the second derivative signal 416 within thebaseline portion 404 of the window 402 that are greater than the maximumthreshold 464, and the number of amplitudes 462 of the second derivativesignal 416 within the baseline portion 404 of the window 402 that areless than the minimum threshold 466, Block 620.

A determination is made as to whether a combined amplitude and baselinecrossings threshold has been satisfied, Block 622, by determining bothwhether a sum of the number of positive zero crossings within thebaseline portion 404 of the window 402 and the number of negative zerocrossings within the baseline portion 404 of the window 402 is greaterthan a baseline crossing threshold, such as four for example, andwhether a sum of the number of amplitudes 460 of the second derivativesignal 416 within the baseline portion 404 of the window 402 that aregreater than the maximum threshold 464, and the number of amplitudes 462of the second derivative signal 416 within the baseline portion 404 ofthe window 402 that are less than the minimum threshold 466 is greaterthan an amplitude threshold, such as 10 samples or 16 microvolts forexample.

If the combined amplitude and baseline crossings threshold has beensatisfied, Yes in Block 622, a noise event is determined, Block 604. Ifthe combined amplitude and baseline crossings threshold has not beensatisfied, No in Block 622, and therefore none of the predeterminedconditions, Blocks 602, 606 610, 614, 618 and 622 are met, a noise eventis not identified for the current characteristic P-wave 400, Block 624.

It is understood that any single one or combination and order of thepredetermined conditions, Blocks 602, 606 610, 614, 618 and 622, may beutilized in determining whether a noise event is identified, andtherefore numerous combinations of the conditions, or single ones of theconditions may be utilized in determining a noise event, and thereforethe disclosure is not limited to the combination and order of theconditions as illustrated in FIG. 8. In this way, a noise event may bedetermined in response to one of any of the conditions of Blocks 602,606 610, 614, 618 and 622.

Therefore, the characteristic signal 400 may be determined to be a noiseevent if any one of the following noise conditions are met:

N1. bwinZCmax>156 ms AND bwinZCmin<−156 ms (Block 602)N2. bwinZCmax>468 ms OR bwinZCmin<−468 ms (Block 606)N3. bwinZCpThr+bwinZCnThr>3 (Block 610)N4. bwinZCpThr+bwinZCnThr=3 AND {4<(bwinZCp+bwinZCn)<10} (Block 614)N5. pwinZCp+pwinZCn>4 (Block 618)N6. bwinZCp+bwinZCn>4 AND bwinPThr+bwinNThr>10 (Block 622)

where bwinZCmax is the maximum amplitude 460 of the second derivativesignal 416 at a positive zero crossing of the first derivative signal414 within the baseline portion 404 of the window 402, bwinZCmin is theminimum amplitude 462 of the second derivative signal 416 at a negativezero crossing of the first derivative signal 414 within the baselineportion 404 of the window 402, and the remaining conditions are asdescribed above.

Thus, an apparatus and method have been presented in the foregoingdescription with reference to specific embodiments. It is appreciatedthat various modifications to the referenced embodiments may be madewithout departing from the scope of the invention as set forth in thefollowing claims.

1. A method of determining noise in response to a cardiac signal in animplantable medical device, comprising: sensing the cardiac signal;determining a sensing window within the sensed cardiac signal, thesensing window comprising a first portion and a second portion;determining a first derivative signal of the sensed cardiac signalwithin only one of the first portion and the second portion of thesensing window; determining a second derivative signal of the sensedcardiac signal within the one of the first portion and the secondportion of the sensing window; determining whether an amplitude of thesecond derivative signal satisfies an amplitude threshold; anddetermining noise in response to the amplitude of the second derivativesignal satisfying the amplitude threshold.
 2. The method of claim 1,wherein the first derivative signal and the second derivative signal aredetermined within the first portion of the sensing window, furthercomprising: determining positive and negative zero crossings of thefirst derivative signal within the first portion; determining whether amaximum amplitude of the second derivative signal at the positive zerocrossings and a minimum amplitude of the second derivative signal at thenegative zero crossings satisfy the amplitude threshold; and determiningnoise in response to both the maximum amplitude and the minimumamplitude satisfying the amplitude threshold.
 3. The method of claim 1,wherein the first derivative signal and the second derivative signal aredetermined within the first portion of the sensing window, furthercomprising: determining positive and negative zero crossings of thefirst derivative signal within the first portion; determining whether amaximum amplitude of the second derivative signal at the positive zerocrossings and a minimum amplitude of the second derivative signal at thenegative zero crossings satisfy the amplitude threshold; and determiningnoise in response to only one of the maximum amplitude and the minimumamplitude satisfying the amplitude threshold.
 4. The method of claim 1,wherein the first derivative signal and the second derivative signal aredetermined within the first portion of the sensing window, furthercomprising: determining positive and negative zero crossings of thefirst derivative signal within the first portion; determining a firstnumber of amplitudes of the second derivative signal at the positivegoing zero crossing that are greater than a first amplitude threshold;determining a second number of amplitudes of the second derivativesignal at the negative going zero crossing that are less than a secondamplitude threshold; determining a sum of the first number of amplitudesand the second number of amplitudes; and determining noise based on thedetermined sum.
 5. The method of claim 4, further comprising:determining whether a total number of the positive zero crossings andthe negative zero crossings is within a zero crossings sum threshold;and determining noise in response to both the determined sum beinggreater than an amplitude sum threshold and the total number of thedetermined positive zero crossings and the negative zero crossings beingwithin the zero crossings sum threshold.
 6. The method of claim 1,wherein the first derivative signal and the second derivative signal aredetermined within the second portion of the sensing window, furthercomprising: determining positive and negative zero crossings of thefirst derivative signal within the second portion; determining whether atotal number of the determined positive zero crossings and the negativezero crossing is greater than a total zero crossings threshold; anddetermining noise in response to the total number of the determinedpositive zero crossings and the negative zero crossing being greaterthan the total zero crossings threshold.
 7. The method of claim 1,wherein the first derivative signal and the second derivative signal aredetermined within the first portion of the sensing window, furthercomprising: determining positive and negative zero crossings of thefirst derivative signal within the first portion; determining whether atotal number of the determined positive zero crossings and the negativezero crossings is greater than a total zero crossings threshold;determining a first number of amplitudes of the second derivative signalat the positive going zero crossings of the first derivative signal thatare greater than a positive going amplitude threshold; determining asecond number of amplitudes of the second derivative signal at thenegative going zero crossings of the first derivative signal that areless than a negative going amplitude threshold; determining whether asum of the first number of amplitudes and the second number ofamplitudes is greater than a total amplitude threshold; and determiningnoise in response to both the total number of the determined positivezero crossings and the negative zero crossings being greater than thetotal zero crossings threshold and the sum of the first number ofamplitudes and the second number of amplitudes being greater than thetotal amplitude threshold.
 8. The method of claim 1, comprising:determining a first noise condition comprising determining whether amaximum amplitude of the second derivative signal at positive zerocrossings of the first derivative signal and a minimum amplitude of thesecond derivative signal at negative zero crossings of the firstderivative signal satisfy a first amplitude threshold; determining asecond noise condition comprising: determining a first number ofamplitudes of the second derivative signal at positive going zerocrossings that are greater than a second amplitude threshold; anddetermining a second number of amplitudes of the second derivativesignal at negative going zero crossings that are less than a thirdamplitude threshold; determining a third noise condition comprisingdetermining a total number of positive zero crossings and negative zerocrossings of the first derivative signal within the first portion of thesensing window; determining a fourth noise condition comprisingdetermining a total number of positive zero crossings and negative zerocrossings of the first derivative signal within the second portion ofthe sensing window; and determining a fifth noise condition comprising:determining a total number of positive zero crossings and negative zerocrossings greater than a total zero crossings threshold; determining afirst number of amplitudes of the second derivative signal at thepositive going zero crossings of the first derivative signal that aregreater than a positive going amplitude threshold; and determining asecond number of amplitudes of the second derivative signal at negativegoing zero crossings of the first derivative signal that are less than anegative going amplitude threshold, wherein noise is determined inresponse to one of the first noise condition, the second noisecondition, the third noise condition, the fourth noise condition and thefifth noise condition.
 9. The method of claim 1, wherein the medicaldevice comprises a subcutaneous device.
 10. An implantable medicaldevice for determining noise within a cardiac signal, comprising: aplurality of electrodes sensing the cardiac signal; and a processorconfigured to determine a sensing window of the sensed cardiac signal,the sensing window comprising a first portion and a second portion,determine a first derivative signal of the sensed cardiac signal withinonly one of the first portion and the second portion of the sensingwindow, determine a second derivative signal of the sensed cardiacsignal within the one of the first portion and the second portion of thesensing window, determine whether an amplitude of the second derivativesignal satisfies an amplitude threshold, and determine noise in responseto the amplitude of the second derivative signal satisfying theamplitude threshold.
 11. The medical device of claim 10, wherein thefirst derivative signal and the second derivative signal are determinedwithin the first portion of the sensing window, the processor furtherconfigured to determine positive and negative zero crossings of thefirst derivative signal within the first portion, determine whether amaximum amplitude of the second derivative signal at the positive zerocrossings and a minimum amplitude of the second derivative signal at thenegative zero crossings satisfy the amplitude threshold, and determinenoise in response to both the maximum amplitude and the minimumamplitude satisfying the amplitude threshold.
 12. The medical device ofclaim 10, wherein the first derivative signal and the second derivativesignal are determined within the first portion of the sensing window,the processor further configured to determine positive and negative zerocrossings of the first derivative signal within the first portion,determine whether a maximum amplitude of the second derivative signal atthe positive zero crossings and a minimum amplitude of the secondderivative signal at the negative zero crossings satisfy the amplitudethreshold, and determine noise in response to only one of the maximumamplitude and the minimum amplitude satisfying the amplitude threshold.13. The medical device of claim 10, wherein the first derivative signaland the second derivative signal are determined within the first portionof the sensing window, the processor further configured to determinepositive and negative zero crossings of the first derivative signalwithin the first portion, determine a first number of amplitudes of thesecond derivative signal at the positive going zero crossing that aregreater than a first amplitude threshold, determine a second number ofamplitudes of the second derivative signal at the negative going zerocrossing that are less than a second amplitude threshold, determine asum of the first number of amplitudes and the second number ofamplitudes, and determine noise based on the determined sum.
 14. Themedical device of claim 13, wherein the processor is further configuredto determine whether a total number of the determined positive zerocrossings and the negative zero crossings is within a zero crossings sumthreshold, and determine noise in response to both the determined sumbeing greater than an amplitude sum threshold and the total number ofthe determined positive zero crossings and the negative zero crossingsbeing within the zero crossings sum threshold.
 15. The medical device ofclaim 10, wherein the first derivative signal and the second derivativesignal are determined within the second portion of the sensing window,the processor further configured to determine positive and negative zerocrossings of the first derivative signal within the second portion,determine whether a total number of the determined positive zerocrossings and the negative zero crossing is greater than a total zerocrossings threshold, and determine noise in response to the total numberof the determined positive zero crossings and the negative zero crossingbeing greater than the total zero crossings threshold.
 16. The medicaldevice of claim 10, wherein the first derivative signal and the secondderivative signal are determined within the first portion of the sensingwindow, the processor further configured to determine positive andnegative zero crossings of the first derivative signal within the firstportion, determine whether a total number of the determined positivezero crossings and the negative zero crossings is greater than a totalzero crossings threshold, determine a first number of amplitudes of thesecond derivative signal at the positive going zero crossings of thefirst derivative signal that are greater than a positive going amplitudethreshold, determine a second number of amplitudes of the secondderivative signal at the negative going zero crossings of the firstderivative signal that are less than a negative going amplitudethreshold, determine whether a sum of the first number of amplitudes andthe second number of amplitudes is greater than a total amplitudethreshold, and determine noise in response to both the total number ofthe determined positive zero crossings and the negative zero crossingsbeing greater than the total zero crossings threshold and the sum of thefirst number of amplitudes and the second number of amplitudes beinggreater than the total amplitude threshold.
 17. The medical device ofclaim 10, wherein the processor is further configured to determine afirst noise condition comprising determining whether a maximum amplitudeof the second derivative signal at positive zero crossings of the firstderivative signal and a minimum amplitude of the second derivativesignal at negative zero crossings of the first derivative signal satisfya first amplitude threshold, determine a second noise conditioncomprising determining a first number of amplitudes of the secondderivative signal at positive going zero crossings that are greater thana second amplitude threshold, and determining a second number ofamplitudes of the second derivative signal at negative going zerocrossings that are less than a third amplitude threshold, determine athird noise condition comprising determining a total number of positivezero crossings and negative zero crossings of the first derivativesignal within the first portion of the sensing window, determine afourth noise condition comprising determining a total number of positivezero crossings and negative zero crossings of the first derivativesignal within the second portion of the sensing window, and determine afifth noise condition comprising determining a total number of positivezero crossings and negative zero crossings greater than a total zerocrossings threshold; determining a first number of amplitudes of thesecond derivative signal at the positive going zero crossings of thefirst derivative signal that are greater than a positive going amplitudethreshold; and determining a second number of amplitudes of the secondderivative signal at negative going zero crossings of the firstderivative signal that are less than a negative going amplitudethreshold, wherein noise is determined in response to one of the firstnoise condition, the second noise condition, the third noise condition,the fourth noise condition and the fifth noise condition.
 18. Themedical device of claim 10, wherein the medical device comprises asubcutaneous device.
 19. A computer-readable medium storing a set ofinstructions which cause a processor of an implantable medical device toperform a method of determining a noise of a cardiac signal, comprising:sensing the cardiac signal; determining a sensing window of the sensedcardiac signal, the sensing window comprising a first portion and asecond portion; determining a first derivative signal of the sensedcardiac signal within only one of the first portion and the secondportion of the sensing window; determining a second derivative signal ofthe sensed cardiac signal within the one of the first portion and thesecond portion of the sensing window; determining whether an amplitudeof the second derivative signal satisfies an amplitude threshold; anddetermining noise in response to the amplitude of the second derivativesignal satisfying the amplitude threshold.
 20. The computer readablemedium of claim 19, wherein the implantable medical device comprises asubcutaneous device.
 21. An implantable medical device for determiningnoise in a cardiac signal, comprising: a plurality of electrodesconfigured to sense the cardiac signal; and a processor configured todetermine a sensing window associated with at least one cardiac event inthe sensed cardiac signal, the sensing window comprising a first portionand a second portion, determine a first derivative signal of the cardiacsignal within the sensing window, determine a second derivative signalof the cardiac signal within the sensing window, identify a noise eventwithin the sensing window based at least on the analysis of amplitudesof the second derivative signal.
 22. The medical device of claim 21,wherein the processor is further configured to determine positive goingand negative going zero crossings in the first derivative signal withinthe first portion of the sensing window, determine amplitudes of thesecond derivative signal at the positive going and negative going zerocrossings, determine a maximum amplitude of the determined amplitudes atthe positive going zero crossings, determine a minimum amplitude of thedetermined amplitudes at the negative going zero crossings, andidentifying the noise event within the sensing window based on at leastone of the maximum amplitude and the minimum amplitude.
 23. The medicaldevice of claim 22, wherein the processor is configured to identify thenoise event within the sensing window when both the maximum amplitude isgreater than a first threshold amplitude and the minimum amplitude isless than a second threshold amplitude.
 24. The medical device of claim23, wherein the processor is configured to identify the noise eventwithin the sensing window when either the maximum amplitude is greaterthan a third threshold amplitude or the minimum amplitude is less than afourth threshold amplitude.
 25. The medical device of claim 24, whereinthe third threshold is greater than the first threshold and the fourththreshold is less than the second threshold.
 26. The medical device ofclaim 21, wherein the processor is further configured to determinepositive going and negative going zero crossings in the first derivativesignal within the first portion of the sensing window, determineamplitudes of the second derivative signal at the positive going andnegative going zero crossings, determine a first number of thedetermined amplitudes of the second derivative signal at the positivegoing zero crossing that are greater than a fifth threshold amplitude,determine a second number of the determined amplitudes of the secondderivative signal at the negative going zero crossing that are less thana sixth threshold amplitude, determine a sum of the first number ofamplitudes and the second number of amplitudes, and identify the noiseevent within the sensing window based on the determined sum.
 27. Themedical device of claim 26, wherein the processor is configured toidentify the noise event within the sensing window when the sum isgreater than a threshold sum.
 28. The medical device of any one of claim27, wherein the processor is further configured to determine a totalnumber of zero crossings within the first portion of the sensing windowby summing the number of determined positive going and negative goingzero crossings within the first portion of the sensing window, andidentify the noise event within the sensing window when both thedetermined sum is equal to the threshold sum and the total number ofzero crossings is within a predetermined range.
 29. The medical deviceof claim 21, wherein the processor is further configured to determinepositive going and negative going zero crossings in the first derivativesignal within the second portion of the sensing window, determine atotal number of the determined positive zero crossings and the negativezero crossings in first derivative signal of the second portion of thesensing window, and identify the noise event within the sensing windowwhen the total number of zero crossings in first derivative signal ofthe second portion of the sensing window is greater than a total zerocrossings threshold.
 30. The medical device of claim 21, wherein theprocessor is further configured to determine positive going and negativegoing zero crossings of the first derivative signal within the firstportion of the sensing window, determine a total number of thedetermined positive going zero crossings and the negative going zerocrossings within the first portion of the sensing window, determine afirst number of amplitudes of the second derivative signal at thepositive going zero crossings within the first portion of the sensingwindow that are greater than a positive going amplitude threshold,determine a second number of amplitudes of the second derivative signalat the negative going zero crossings within the first portion of thesensing window that are less than a negative going amplitude threshold,determine whether a sum of the first number of amplitudes and the secondnumber of amplitudes is greater than a threshold number, and identifythe noise event in the sensing window when both the total number of thedetermined positive zero crossings and the negative zero crossings aregreater than the total zero crossings threshold and the sum of the firstnumber of amplitudes and the second number of amplitudes are greaterthan the threshold number.
 31. The medical device of claim 21, whereinthe medical device comprises a subcutaneous device.
 32. The medicaldevice of claim 21, wherein the sensing window is a P-wave sensingwindow, the first portion is a baseline portion and the second portionis a P-wave portion.